Leo77,
I believe that JT was referring to control problems rather than overboosting the engine.
Maxrate172,
Just about every airliner uses some form of derating power, this can be either "fixed derates" or "flex (Boeing's Assumed Temperature Method)". I'm presuming that Old Smokey is referring to a B747-400 with Trent engines, combing both the fixed derate of 20% plus the assumed temperature reduction of 25% you end up with that aircraft taking-off with approx 65% of the available power!
The purpose of this is to enhance engine life.
Mutt.